Multicolor printing apparatus

Abstract

In a multicolor printing apparatus in which a laser beam is scanned by a rotary polygon mirror to form an electrostatic latent image on a photosensitive medium, multicolor printing is effected by forming images of individual plural colors. When plane mirrors of the rotary polygon mirror which are used for scanning to form the head lines of images of different colors differ from each other, the difference in plane inclination between plane mirrors causes color shift or misregistration and image deformation. In order to prevent this disadvantage, the same mirror plane of the rotary polygon mirror is managed to be used for scanning to form the head lines of images of different colors.

We claim: 
     
       1. A multicolor printing apparatus for printing a color picture having a plurality of color components on a transfer medium by forming an electrostatic latent image corresponding to one of said plurality of color components by a laser beam, developing the electrostatic latent image to produce a developed image, superimposing the developed image on said transfer medium, and iterating for all others of said plurality of color components to form the color picture, said multicolor printing apparatus comprising: a rotatable photosensitive medium for forming an electrostatic latent image of one after another of said plurality of color components of said picture on a same position at every new rotation;   light emitting means for emitting said laser beam;   a rotary polygon mirror having a plurality of plane mirror surfaces for scanning the laser beam from said light emitting means in a direction orthogonal to a direction of rotation of said photosensitive medium;   first detection means for detecting a first timing when said photosensitive medium reaches a predetermined position;   second detection means for detecting a second timing when a predetermined one of said plurality of plane mirror surfaces of said rotary polygon mirror reflects said laser beam, said second detection means including a photosensor located at a fixed position for sensing laser beams reflected from said plurality of plane mirror surfaces to provide a sensing signal and means for recognizing said second timing based on said sensing signal; and   control means for controlling a rotation speed of said rotary polygon mirror in accordance with a difference between said first and second timings so that the laser beam for the first scanning line in every one of said plurality of color components of the picture is reflected on a same lane mirror surface.   
     
     
       2. A multicolor printing apparatus according to claim 1 wherein said control means is operative to make a rotational phase of said photosensitive medium coincident to a rotational phase of said rotary polygon mirror for each of a plurality of different colors before said photosensitive medium reaches a printing start position. 
     
     
       3. A multicolor printing apparatus according to claim 1 further comprising an image signal deliverer responsive to a signal from said control means indicating that said rotary polygon mirror rotates at a predetermined speed to deliver image data. 
     
     
       4. A multicolor printing apparatus for printing a color picture having a plurality of color components on a transfer medium by forming an electrostatic latent image corresponding to one of said plurality of color components by a laser beam, developing the electrostatic latent image to produce a developed image, superimposing the developed image on said transfer medium, and iterating for all others of said plurality of color components to form the color picture, said multicolor printing apparatus comprising: a movable photosensitive medium for forming an electrostatic latent image of one after another of said plurality of color components of said picture on a same position;   light emitting means for emitting said laser beam;   a rotary polygon mirror having a plurality of plane mirror surfaces for scanning the laser beam from said light emitting means in a direction orthogonal to a direction of movement of said photosensitive medium;   first detection means for detecting a first timing when said photosensitive medium reaches a predetermined position;   second detection means for detecting a second timing when a predetermined one of said plurality of plane mirror surfaces of said rotary polygon mirror reflects said laser beam, said second detection means including a photosensor located at a fixed position for sensing laser beams reflected from said plurality of plane mirror surfaces to provide a sensing signal and means for recognizing said second timing based on said sensing signal; and   control means for controlling a rotation speed of said rotary polygon mirror in accordance with a difference between said first and second timings so that the laser beam for the first scanning line in every color component of the picture is reflected on a same plane mirror surface.   
     
     
       5. A multicolor printing apparatus according to claim 4 wherein said control means is operative to make a movement phase of said photosensitive medium coincident to a rotational phase of said rotary polygon mirror for each of a plurality of different colors before said photosensitive medium reaches a printing start position. 
     
     
       6. A multicolor printing apparatus according to claim 4 further comprising an image signal deliverer responsive to a signal from said control means indicating that said rotary polygon mirror rotates at a constant speed to deliver image data. 
     
     
       7. A multicolor printing apparatus for printing a color picture having a plurality of color components on a transfer medium by forming an electrostatic latent image of a first one of said plurality of color components by a laser beam, developing the electrostatic latent image to produce a developed image, superimposing the developed image on said transfer medium, and iterating for second and ensuing ones of said plurality of color components to form the color picture, said multicolor printing apparatus comprising: a movable photosensitive medium;   light emitting means for emitting said laser beam;   a rotary polygon mirror for scanning the laser beam from said light emitting means in a direction orthogonal to the direction of movement of said photosensitive medium;   first detection means for detecting a timing when said photosensitive medium reaches a predetermined position; and   timing control means operable to control the generation of light by said light emitting means after said timing detected by said first detection means, so as to start the formation of an image of said first one of said plurality of color components on said photosensitive medium and to start the formation of images of said second and ensuing ones of said plurality of color components from a plane mirror of said rotary polygon mirror at which the formation of the image of first color is started.   
     
     
       8. A multicolor printing apparatus according to claim 7 wherein said timing control means includes a corrector for changing the rotation speed of said rotary polygon mirror before the formation of the image of second color is started. 
     
     
       9. A multicolor printing apparatus according to claim 7 wherein said photosensitive medium has the form of an endless belt. 
     
     
       10. A multicolor printing apparatus for printing a color picture having a plurality of color components on a transfer medium by forming an electrostatic latent image corresponding to one of said plurality of color components by a laser beam, developing the electrostatic latent image to produce a developed image, superimposing the developed image on said transfer medium, and iterating for all others of said plurality of color components to form the color picture, said multicolor printing apparatus comprising: a movable photosensitive medium for forming an electrostatic latent image of one after another of said plurality of color components of said picture on a same position;   light emitting means for emitting said laser beam;   a rotary polygon mirror having a plurality of plane mirror surfaces for scanning the laser beam from said light emitting means in a direction orthogonal to a direction of movement of said photosensitive medium;   first detection means for detecting a first timing when said photosensitive medium reaches a predetermined position;   second detection means for detecting a second timing when a predetermined one of said plurality of plane mirror surfaces of said rotary polygon mirror reflects said laser beam, said second detection means including a photosensor located at a fixed position for sensing laser beams reflected from said plurality of plane mirror surfaces to provide a sensing signal and means for recognizing said second timing based on said sensing signal;   comparison means for determining a time difference between said first timing and said second timing;   control means for controlling a rotation speed of said rotary polygon mirror in accordance with a result of comparison by said comparison means so that the laser beam for the first scanning line in every one of said color components of the picture is reflected on a same plane mirror surface; and   means for driving said light emitting means so as to form image data individually of individual ones of said plurality of color components under a condition that said rotary polygon mirror is controlled by said control means to rotate at a predetermined speed.   
     
     
       11. A multicolor printing apparatus according to claim 10 wherein said photosensitive medium has the form of an endless belt having a hole at a predetermined position, and said first detection means detects said hole. 
     
     
       12. A multicolor printing apparatus for printing a color picture having a plurality of color components including first, second and ensuing color components on a transfer medium by forming an electrostatic latent image corresponding to a first one of said color components by a laser beam, developing the electrostatic latent image to produce a developed image, superimposing the developed image on said transfer medium, and iterating for second and ensuing ones of said color components to form the color picture, the multicolor printing apparatus comprising: a movable photosensitive medium for forming an electrostatic latent image of one color component after another of the picture on a same position;   light emitting means for emitting a laser beam;   a rotary polygon mirror having a plurality of plane mirror surfaces for scanning the laser beam from said light emitting means in a direction orthogonal to a direction of movement of said photosensitive medium;   first detection means for detecting a first timing when said photosensitive medium reaches a predetermined position;   second detection means for detecting a second timing when a predetermined one of said plurality of plane mirror surfaces of said rotary polygon mirror reflects said laser beam, said second detection means including a photosensor located at a fixed position for sensing laser beams reflected from said plurality of plane mirror surfaces to provide a sensing signal and means for recognizing said second timing based on said sensing signal;   comparison means for determining a time difference between said first timing and said second timing;   control means for controlling a rotation speed of said rotary polygon mirror in accordance with a result of comparison by said comparison means before commencing formation of images so as to start formation of images of the second and ensuing color components onto said photosensitive medium from a same plane mirror surface of said rotary polygon mirror at which formation of an image of the first color component is started; and   means for driving said light emitting means so as to form image data individually of individual ones of said plurality of color components.   
     
     
       13. A multicolor printing apparatus for printing a color picture having a plurality of color components including first, second and ensuing color components on a transfer medium by forming an electrostatic latent image corresponding to a first one of said plurality of color components by a laser beam, developing the electrostatic latent image to produce a developed image, superimposing the developed image on said transfer medium, and iterating for second and ensuing ones of said plurality of color components to form the color picture, said multicolor printing apparatus comprising: a movable photosensitive medium for forming an electrostatic latent image of one after another of said plurality of color components of said picture on a same position;   a light emitting means for emitting said laser beam;   a rotary polygon mirror having a plurality of plane mirror surfaces for scanning the laser beam from said light emitting means in a direction orthogonal to a direction of movement of said photosensitive medium;   first detection means for detecting a timing when said photosensitive medium reaches a predetermined position and providing a first detection signal;   second detection means for providing a second detection signal only when a predetermined one of said plurality of plane mirror surfaces of said rotary polygon mirror reflects said laser beam, said second detection means including a photosensor located at a fixed position for sensing laser beams reflected from said plurality of plane mirror surfaces to provide a sensing signal and means for producing said second detection signal based on said sensing signal;   comparison means for determining a time difference between said first detection signal from said first detection means and said second detection signal from said second detection means;   control means for controlling a rotation speed of said rotary polygon mirror in accordance with a result of comparison by said comparison means before commencing formation of images so as to start formation of images of the second and ensuing ones of said color components onto said photosensitive medium from a same plane mirror surface of said rotary polygon mirror at which formation of an image of the first color component is started; and   means for driving said light emitting means so as to form image data individually of individual ones of said plurality of color components.